This invention relates generally to electro-optical systems and more particularly to electro-optical systems having a plurality of gain media to produce a composite, coherent beam of energy.
As is known in the art, electro-optical systems have a wide range of applications. Many of these applications include the use of a plurality of gain media to produce a composite, coherent beam of energy. Some of these applications are medical, and defense applications where it is often desirable to utilize high power laser electro-optical systems. The development of high power laser systems such as chemical lasers and gas lasers, has achieved some significant levels of laser power. These applications include metal cutting and welding, medical procedures such as Transmyocardial Revascularization (TMR) to treat coronary artery disease, direct printing and engraving, and defense weapons. However, many laser systems currently used in these applications remain complex, cumbersome and have drawbacks such as low efficiency, and the requirement for non-renewable energy sources. These laser systems are not readily scalable to higher power ranges.
If high power laser systems are to become more effective and widespread, new laser technologies are required that provide more compact, efficient, stable and higher powered than systems currently available. For example, diode-pumped fiber lasers have demonstrated efficient electrical-to-optical power conversion into a diffraction-limited laser beam. Single mode outputs of over 100 W have been reported from a fiber laser pumped by semiconductor diode lasers. The conversion of the multimode output of the diode laser pump to the single mode fiber output can be achieved with a quantum efficiency of approximately 85%, resulting in an overall efficiency of the order of 30%. These individual lasers can be constructed to be rugged and compact. Their electric energy source is widely available, easily renewed, and can be generated by many different techniques in any environment including space or under water. However, intrinsic characteristics such as non-linear effects inside the small single-mode core of the fiber ultimately limit the output power generated individually by such devices. The power output of an individual diode-pumped fiber laser will ultimately be limited by the damage threshold of the fiber core and cladding materials, as well as facet coatings.
In order to circumvent these limitations and generate scalable high output power levels in a diffraction-limited beam, laser systems can coherently combine the output of several fiber lasers. As is known in the art, the technique of phase locking allows combining several optical beams into a single beam. This technique combines the output power of each individual optical beam while preserving the spatial and spectral coherence of each individual beam. This approach enables the scalability of laser systems that can produce high power coherent diffraction-limited beams.
Coherent combination of multiple beams can be achieved by several different techniques. These techniques require that all the beams have the same wavelength, the same polarization, and be phase locked in the proper phase state in the plane of combination. In a laser system, this can be accomplished in several different ways. For example, one can use a single beam to xe2x80x9cseedxe2x80x9d or injection-lock all the laser beams to be combined, thus ensuring that they be all at the same wavelength and polarization, and that a stable coherent phase regime exists for each source. One then needs to control or adjust the phases of all the laser beams to achieve coherent combination. Laser beams can also be combined in series with a nearest neighbor using the Talbot effect as in known in the art.
Another technique is to optically couple all the laser beams together in parallel in a common cavity, thus ensuring that they are all at the same frequency and in a single coherent phase state. External-cavity coupling of laser diodes has been successfully demonstrated to produce output beams both spectrally coherent as well as spatially coherent.
Many conventional external cavity laser designs utilize a xe2x80x9c4-Fxe2x80x9d optical configuration. The xe2x80x9c4-Fxe2x80x9d refers to the four focal length size of the external cavity. This technique uses two lenses inside the external cavity to Fourier transform the optical input pattern (i.e. electric field amplitude) a total of 4 times in a single round trip, resulting in an identity operation as described in xe2x80x9cIntroduction to Fourier Opticsxe2x80x9d, by Joseph W. Goodman published by Mcgraw Hill Book Company, 1968. A spatial filter is placed at the Fourier plane and only allows light intensity (magnitude squared of the Fourier Transform of the input electric field pattern) which is coherent between the multiple gain elements to pass through the filter. The radiation beam must pass through the spatial filter twice before reentering the gain media. The double pass through the spatial filter reduces the overall efficiency of the laser array. The 4-F designed laser arrays require polarization adjusters, phase adjusters and the spatial filter in the path of the radiation. Some of the problems associated with these designs include their large size, complexity, the need for numerous adjustments and stability of the laser output.
Thus, in high power laser applications, there is a need for a system to efficiently combine multiple laser sources using a compact external cavity design to provide a stable phased locked output without requiring numerous adjustments.
In accordance with the present invention, an electro-optical system is provided having a plurality of electromagnetic gain media having media having a corresponding plurality of apertures disposed in a predetermined spatial distribution. The system further includes a refracting surface disposed to intercept energy from the plurality of apertures and a partially reflecting surface to direct a portion of the intercepted energy back toward the plurality of apertures with the directed energy being distributed about the plurality of apertures. The predetermined spatial distribution of the apertures is selected to provide a spatial filter to establish a composite beam comprised of energy from the plurality of gain media with phase coherency.
In accordance with one feature of the invention, an electro-optical system is provided having a plurality of apertures for a plurality of electromagnetic gain media. The apertures have spacings therebetween, and the spacings are disposed in a predetermined spatial distribution. The system further includes a common element disposed to intercept energy from the apertures media and direct the energy back to the plurality of gain media through the apertures. The directed energy is distributed about the plurality of apertures with the spacings providing a spatial filter to establishing a composite beam comprised of energy from the plurality of gain media, such beams in the composite beam having phase coherency.
With such an arrangement, the cavity is shorter, more compact, more stable, and much simpler than many traditional external cavity architectures, because the external cavity requires only half of the focal length (f) of the common element as the total length of the external cavity and does not require a dedicated spatial filter element.
In accordance with a further aspect of the present invention, a method to produce a phase coherent beam includes the steps of providing a plurality of apertures for a plurality of electromagnetic gain media producing a plurality of radiation beams. A refracting surface and a partially reflecting surface are disposed to reflect portions of the radiation. The method includes spacing entrance apertures of each of the plurality of electromagnetic gain media apart to provide a predetermined spatial distribution. The spatial distribution of spacing between the apertures provides a spatial filter and establishing a composite beam comprised of energy from the plurality of gain media with phase coherency. With such a technique, the outputs of a spatially separated multitude of laser or gain media outputs can be coherently combined into a composite beam and many types of laser or gain media source can be coupled together.
In accordance with a further aspect of the present invention, an electro-optical system is provided having a plurality of electromagnetic gain media having apertures disposed in a predetermined spatial distribution. The system further includes a common element disposed to intercept energy from the plurality of electromagnetic gain media and form an interference pattern across the apertures, such interference pattern having regions of constructive interference and regions of destructive interference. The regions of constructive interference are disposed on the apertures producing a composite beam comprised of energy from the plurality of gain media with phase coherency. With such an arrangement, energy from the gain media in the regions of constructive interference is fed back into the individual apertures of the gain media. The energy feedback into the gain media provides output energy having a coherent phase relation with a portion of the energy in the gain media and a cavity of the electro-optical system without requiring a dedicated spatial filter element.
In one embodiment, an assembly having a cavity uses a plurality of optical energy gain media with apertures spaced apart to provide an input optical pattern which is a very close match to the Fourier Transform of the input optical pattern when the elements are phase locked together and emitting radiation as a coherent array. The cavity and optical input pattern are used to optically fold the optical gain media beam back on itself in such a manner as to couple energy amplified by the plurality of optical gain media together and to provide phase locking of the energy amplified by the plurality of gain media. Therefore optical gain media are spaced apart by a predetermined spatial distribution in the assembly, and more particularly the associated apertures perform the role of a spatial filter in the cavity.